Throughout this application various publications are referenced by arabic numerals within parentheses. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
The healing of wounds and the effect of oxygen tension has been intensively studied (1). Among the components important in the healing process are fibroblast proliferation, angiogenesis, collagen synthesis, and reepithelialization.
Soon after injury, whether accidental or surgically induced, undifferentiated mesenchymal cells transform to migratory fibroblasts, which migrate into and across the injured wound. It is known that fibroblasts are aerobic in nature. Fibroblasts are stimulated to produce collagen. While experiments from cultured fibroblasts suggest that high lactate and ascorbic acid concentration typical of hypoxic conditions may activate some of the fibroblast collagen-synthesizing enzymes, animal studies involving low, normal, and high oxygen tensions nevertheless demonstrate increased rates of collagen synthesis under hyperoxic rather than hypoxic conditions.
Angiogenesis, on the other hand, appears to be stimulated by a hypoxic tissue gradient, with new capillaries extending in the direction of lower oxygen concentration. When a hypoxic gradient no longer exists, angiogenesis is minimized or static. Epithelialization is also known to be related to oxygen tension, with higher rates of epithelial proliferation observed under hyperoxic as opposed to hypoxic conditions.
The supply of oxygen to healing wound tissue may be derived from three sources: oxygen chemically bound to hemoglobin in whole blood; oxygen dissolved in plasma; and oxygen which diffuses into plasma or tissue from the exterior. In deep wounds, the latter is of little importance. The studies of R. P. Gruber et al., for example, indicate that oxygen tension, measured polarographically, increases markedly at 3 bar of 100% O2 in the superficial dermis (0.30–0.34 mm), while the relative oxygen concentration of the deep dermis (1.8–2.2 mm) is unchanged under the same conditions (2).
In surface wounds, all sources of oxygen are important. In wounds of large surface area, however, for example ulcers, only the tissue at the edges of the ulcer or at its base are well supplied with blood, and the growing granulation tissue, in the absence of oxygen diffusing from the exterior, must be supplied by diffusion from blood vessels and plasma, a relatively inefficient process.
It is well established, also, that occlusive coverings that maintain a moist environment promote wound healing (3). Furthermore, it is well known that the changing of wound dressings may interfere with the healing process by disrupting the healing tissue where granulation and collagen synthesis has not imparted sufficient tensile strength to avoid rupture upon dressing removal. However, due to the inability of the blood and plasma to supply optimal oxygen concentration, and due to the further reduction in oxygen from the exterior brought about by the presence of the occluding dressing, a hypoxic condition may rapidly be reached. Although this condition may encourage angiogenesis, it negatively affects collagen synthesis and epithelialization. Moreover, various clostridium species, e.g., C. perfringens and C. septicum, are induced to germinate under hypoxic conditions, which can also support other anaerobic flora (4). In addition to minimizing anaerobic flora by discouraging germination, hyperoxic conditions are known to reduce the concentration of other pathogens as well.
Past treatment of chronic ulcers and gangrenous tissue has, in many cases, involved extensive debridement in combination with antibiotics and systemic hyperbaric oxygen. Room size hyperbaric oxygen chambers or chambers sized for the individual patient have employed pure oxygen at pressures of 2 to 3 bar. Treatment time is limited, as oxygen toxicity and central nervous system (CNS) disorders may result from the increased oxygen content of the blood. Such treatments have met with a great deal of success, but the success may not be due to the increased systemic blood and plasma-derived oxygen supply. The blood and plasma already contain sufficient oxygen for the healing process. Rather, it is the diffusion-limited access of oxygen to the wound that limits the oxygen supply required for optimal healing and minimization of infection. The increased oxygen tension in the wound most likely results directly from increased diffusion into the wound surface from the oxygen in the chamber. Gruber, for example, indicates that rate of oxygen absorption from the skin is roughly proportional to oxygen concentration from nearly 0% to 30% (2). Gruber further indicates, however, that oxygen absorption tends to level off at higher oxygen concentrations.
Due to the expense of large hyperbaric chambers and the systemic effects of oxygen toxicity that they may engender, topical hyperbaric chambers have been proposed. Topical chambers operating at “normal” hyperbaric pressures of 2–3 bar are difficult to seal to the body or extremity being treated, however, without interfering with blood supply to the wound locus. Thus, hyperbaric chambers operating at only modestly elevated pressure have been manufactured, such as a device operating at 22 mm Hg pure oxygen (1.03 bar) (5). However, such chambers are expensive and difficult to sterilize (6). Cross-infection is stated to be common.
Heng and others have proposed a simple hyperbaric oxygen treatment chamber consisting of a polyethylene bag that may be secured to the body or extremity with adhesive tape (6), or a transparent nylon bag with straps and VELCRO® closures (7). Pressure is maintained at between 20 mm Hg and 30 mm Hg. However, the leakage associated with the sealing of such bags requires a relatively high rate of oxygen flow. Thus, this method is useful only in facilities with sufficient oxygen supply, or in controlled home environments where a large oxygen tank is permissible. A disposable hyperbaric treatment bag with improved closure is disclosed in U.S. Pat. No. 5,029,579. Another disposable hyperbaric treatment bag is disclosed in U.S. Pat. No. 5,478,310.
In U.S. Pat. No. 4,875,483, a combination layered dressing having an external low oxygen-permeability layer and an abutting internal oxygen permeable layer has been proposed. The relatively low permeability exterior layer is left attached for 3 to 72 hours creating hypoxia, and hopefully stimulating angiogenesis, following which this layer is removed. However, although the remaining, and now exterior layer is oxygen permeable, the layer nevertheless decreases oxygen transport, and thus hyperbaric treatment, by one of the methods previously described, may be necessary to elevate oxygen levels sufficiently to provide optimal healing.
Ischemia compromises wound healing and wounds in aging populations are more ischemic than those in younger populations (8). It has been demonstrated in ischemic rabbit ear models that topical or hyperbaric oxygen can convert a non-healing wound into a healing wound, and that growth factors (PDGF) provide a synergistic benefit when used with oxygen (9).
It is well known that the speed of epidermal migration on the normal wound is critically dependent on the amount of oxygen available, and this is the rate-limiting step. The control of the local environment is dependent on the local blood supply and the diffusion of oxygen from the atmosphere. Any form of treatment that encourages an increase in the wound fluid and reduces the time during which the wound is non-perfused will tend to increase the rate of healing (10, 11).
It is generally agreed that the tissue surrounding a wound does not alone supply sufficient oxygen for wound repair, and that atmospheric oxygen is required for the formation of hydroxyproline, a key element in epidermal wound healing. It has been demonstrated that 93% of the oxygen incorporated into the hydroxyl groups of newly synthesized hydroxyproline is derived from the atmosphere (12).
It is further generally known that it is likely that oxygen reaches the epidermal cells directly by diffusion through the scab rather than via the vascular or tissue supply. Prior studies of wounds covered with plastic films found that the higher the oxygen permeability of the film, the greater the healing rate (13, 14). Furthermore, the films prevented scab formation, thereby altering the mode of epidermal regeneration. The use of wound dressings that prevent scab formation and have increased oxygen permeability are thought to improve wound healing. The increased presence of oxygen speeds the re-establishment of epithelial continuity. Direct access of pure oxygen to open wounds promotes epidermal cell migration.
Kaufman et al. showed a continuum in wound healing improvement when changing humidified oxygen levels from 21 to 60, and 80–96% on full thickness burns on guinea pigs (15). Niinikoski also suggested that collagen accumulation in the dead space of animal wounds increases with oxygen concentration of the environment, peaking at 70% (16).
A review of topical oxygen and burn wound healing states that oxygen is essential for the contraction, the dominant healing process (17). Topical oxygen has also been shown to improve the healing rate of skin ulcers and wounds where an inadequate supply of oxygen results from peripheral vascular disease or local injury to the microcirculation. Fischer showed topical hyperbaric oxygen treatment improved epithelialization and contraction of decubitus ulcers (5).
Utkina demonstrated that moderate increases in oxygen levels at normal atmospheric pressure increases the closure rate of open wounds (18). He showed healing rate improved with continuous exposure to 45%.
A number of patents have been issued that disclose the use of local generation of oxygen at the wound site to treat wounds in bandage systems using chemical reactions, oxygen saturated solutions, or electrochemical generators (see U.S. Pat. Nos. 5,855,570, 5,578,022, 5,788,682, 5,792,090 and 6,000,403). These concepts have not been commercialized. The present invention allows for gas to be contained simply into the wound dressing, which creates a wound environment with continuous exposure to preset oxygen levels, without need for a gas source such as a generator, saturated solution or a chemical reaction. Since the amount of oxygen consumed by metabolic processes in the wound is relatively small, the materials for the dressing and the volume of the oxygen cavity in the dressing can be selected to maintain the desired oxygen concentration for the practical life of the dressing.
Prior to this invention, larger amounts of oxygen were believed to be required to benefit wound healing, which justified the need for an oxygen releasing source However, the actual amount of oxygen that the wound consumes in cell metabolism is quite small, and simply requires a design that assures a large diffusion gradient for oxygen into the wound during the healing period. Hyperbaric approaches that use elevated pressure to further enhance the oxygen diffusion gradients to transfer more oxygen into the tissue are only used briefly, and once the patient is withdrawn from the high-pressure environment, the oxygen levels in the wound drop down to pre-exposure limits quickly. The present invention operates as a hyperoxic environment without the need for using elevated pressure to create the oxygen diffusion gradient.
Supplying oxygen to a wound on a continuous and ambulatory basis is of benefit to speed healing and reduce infection. The oxygen dressing described below can be complimentary to other therapies and can address a rate-limiting step for various types of wounds.